The invention concerns a method for the characterization of subjective speckle formation.
Speckle patterns are irregular fine-grained light distributions which occur when optically rough projection surfaces are illuminated with sufficiently coherent light, especially laser light. The optically rough projection surfaces have unevenness on the order of magnitude of the wavelength of the light. The speckle pattern arises in the imaging of a light spot on the projection surface and by interference of the light waves scattered at the unevenness of the projection surface. This produces a spatial structure with randomly distributed intensity maxima and intensity minima, having the stochastic fine structure of the reflecting projection surface. The contrast of the speckle grains is determined by the coherence of the wavelength and the roughness of the projection surface. The speckle pattern disappears when the wavelength of the light decreases significantly below the mean roughness of the projection surface. The size of the speckle grains is dependent on the size of the light spot. The larger the light spot, the more fine the granularity of the speckle pattern.
Lasers are being used increasingly as a light source for optical imaging methods, and because of their high optical efficiency they are especially useful in producing bright pictures with a large color space and high contrast. It is possible to achieve a high image quality with unlimited picture size. The laser beam is scanned across a projection surface in order to show the picture being represented. This so-called laser scanning projection is a highly promising technology for use as a picture generation unit, or PGU, in vehicle applications, such as contact-analog head-up displays.
Head-up displays are arranged, for example, in the region of the front windshield of a motor vehicle. By a head-up display, or HUD, is meant a display system in which the user can maintain the posture of his head or the viewing direction in the original orientation when viewing the displayed information, since the information is projected into the field of vision. HUDs generally have an imaging unit, which generates an image, an optical module, and a projection surface. The optical module directs the image onto the projection surface, which is configured as a light-transparent mirror-reflecting windshield. The driver of the vehicle sees the reflected information of the imaging unit and at the same time the actual surroundings behind the windshield.
Speckle patterns arise during projection methods with lasers or other coherent light sources, which on account of the small picture elements are generally coarse grained and perceived by the viewer as an annoyance.
The prior art offers a few possibilities of preventing or reducing the formation of speckle patterns.
One possibility is presented in US 2012 0200832 A1 for reducing speckle patterns in an image projection unit. Here, a convex shaped lens is arranged in front of the light source, which can be placed at variable distance from the light source. Depending on the thus adjustable focal length of the lens, the coherent light is altered so that a speckle pattern is reduced.
In DE 10 2010 002 161 A1 another method is proposed for reducing speckle patterns. Here, a continuously rotating scattering disk is arranged in front of a coherent light source. In this way, various statistical phase variations are imposed on the light waves, so that various subjective noise components are generated during a period of exposure and a noise-reduced intensity pattern thanks to the time averaging over the period of exposure of the individual noise components is detected by two cameras.
The difficulty of objectively quantifying the speckle intensity resides in the high dependency of the measurement results on the measurement system used. No method for speckle quantification is known in the prior art that makes it possible to objectively assess the picture quality in regard to disruptive spot patterns.